In solar cells, when energy particles, called photons, included in solar rays hit an i-layer, electrons and holes are generated by the photovoltaic effect. The electrons move toward an n-layer. The holes move toward a p-layer. The electrons produced by the photovoltaic effect are extracted by an upper electrode and a rear surface electrode. Thereby, light energy is converted into electric energy.
FIG. 11 is a schematic cross-sectional view of an amorphous silicon solar cell. In a solar cell 100, there are stacked in order from top to bottom in the figure: a glass substrate 101 that forms a surface of the solar cell; an upper electrode 103 made of a zinc-oxide-based transparent conductive film that is provided on the glass substrate 101; a top cell 105 made of amorphous silicon; an intermediate electrode 107 made of a transparent conductive film; a bottom cell 109 made of microcrystalline silicon, a buffer layer 110 made of a transparent conductive film; and a rear surface electrode 111 made of a metal film (for example, see Patent Document 1). The intermediate electrode 107 is provided between the top cell 105 and the bottom cell 109.
The top cell 105 is made of three layers of: a p-layer 105p, an i-layer 105i, and an n-layer 105n. The i-layer 105i is formed of amorphous silicon. Furthermore, the bottom cell 109 is also made of three layers of: a p-layer 109p, an i-layer 109i, and an n-layer 109n, similarly to the top cell 105. Of these, the i-layer 109i is made of microcrystalline silicon.
In the solar cell 100 like this, solar light incident from the glass substrate 101 side passes through the upper electrode 103, the top cell 105 (p-i-n-layer), and the buffer layer 110, and is then reflected off the rear surface electrode 111. To improve the conversion efficiency of the solar cell for light energy, a method is adopted such as causing the solar light to reflect off the rear surface electrode 111 or providing the upper electrode 101 with a texture structure. The texture structure has both of a prism effect of extending the optical path of the incident solar light and an effect of enclosing the light. The buffer layer 110 has an object to prevent diffusion of the metal film used for the rear surface electrode 111 and other objects (for example, see Patent Document 2).
In the device structures of different solar cells, different wavelength bands are used for the photovoltaic effect. However, in any of the solar cells, requirements for the transparent conductive film used as the upper electrode include a property of transmitting light for absorption in the i-layer and an electrical conduction property allowing the electrons generated by the photovoltaic effect to be extracted. As the transparent conductive film for the upper electrode, an FTO thin film in which SnO2 is doped with fluorine as impurity or a ZnO-based oxide thin film is used. As for the buffer layer, the requirements include a property of transmitting light reflected off the rear surface electrode for absorption in the i-layer and light reflected off the rear surface electrode, and an electrical conduction property for moving the holes to the rear surface electrode.
Properties required for the transparent conductive films for use in solar cells include roughly three elements of: (1) conductivity property, (2) optical property, and (3) a texture structure. As for (1) conductivity property, low electrical resistance is required in order to extract the generated electricity. Typically, the FTO (fluorine-doped tin oxide) used for the transparent conductive film for a solar cell is a transparent conductive film fabricated by CVD method. The FTO obtains conductivity property by doping SnO2 with F, so that F is replaced by O. Furthermore, a ZnO-based material which receives much attention as a post-ITO (indium tin oxide) is capable of being deposited by sputtering. By doping a material including oxygen defect and Al or Ga to the ZnO, conductivity property can be obtained.
As for (2) optical property, an optical property of transmitting the wavelength band that is absorbed in an electric power generation layer is required because a transparent conductive film for a solar cell is mainly used on the incident light side.
As for (3) texture structure, a texture structure that disperse light in order to efficiently absorb solar light in the electric power generation layer is required. Normally, a ZnO-based thin film fabricated by a sputtering process has a flat surface. Therefore, a treatment of forming a texture by wet etching or the like is required.
However, in the case of forming a transparent conductive film for a solar cell by sputtering and subsequently wet etching a ZnO-based material, carbon-based contamination is generated while etching is performed using a typical oxalic acid-based etching solution or the like as an etching solution for amorphous ITO. The contamination degrades a series resistance of the solar cell, and resulting in decrease in conversion efficiency of the solar cell.